1. Field of the Invention
The present invention relates to a short-circuit protection circuit and method for preventing the risk of a battery pack generating heat etc. caused by short-circuiting of the charging terminals of the battery pack, which is used in equipment such as portable telephone terminals.
2. Description of the Related Art
Typically, portable telephone terminals are provided with a battery pack constituting a part of the portable telephone terminal and mounted in a detachable condition; the terminal is operated using the power supplied from this battery pack.
FIG. 6 is a view showing the layout of a portable telephone terminal 50 on which is mounted a conventional battery pack 70, and a charging/discharging device 60.
In FIG. 6, portable telephone terminal 50 is arranged such that power supplied from battery pack 70 is input through terminals CC71b and CC72b; this input power is used by power source circuit 59 to generate the necessary power for the various units constituting portable telephone terminal 50, this power which is thus generated being supplied to the various units constituting portable telephone terminal 50. That is, this portable telephone terminal 50 is operated by the power supplied from power source circuit 59 using the power supplied from battery pack 70.
For example, the voice signal that is input from microphone 54 is modulated on the local signal from frequency synthesizer 55 and amplified by transmission unit 53, being converted to a radio frequency signal; this radio frequency signal is then transmitted through antenna duplexer circuit 52 and antenna 51.
Also, the radio frequency signal received through antenna 51 and antenna duplexer circuit 52 is amplified and demodulated using the local signal from frequency synthesizer 55, being thereby converted to a voice signal, which voice signal is then output from speaker 57.
Control circuit 58 controls transmission unit 53, reception unit 56 and frequency synthesizer 55 etc.
This control circuit, transmission unit 53, reception unit 56, and frequency synthesizer 55 etc. are then operated by the power supplied from power source circuit 59 using the power supplied from battery pack 70.
Battery pack 70 comprises: a battery cell 71, an over-current cut-off switch 72 connected in series with battery cell 71 and that cuts this off if the current flowing in battery cell 71 is excessive, a thermistor 73 for measuring the temperature inside battery pack 70, a plus terminal CC71a and a minus terminal CC72a for supplying power from battery cell 71 to the power source circuit 59 of portable telephone terminal 50, a plus charging/discharging terminal C71a connected to charging/discharging device 60, a minus charging/discharging terminal C74a, a type-identifying terminal C72a for identifying the type of the capacity of battery pack 70, and a temperature terminal C73a for extracting the temperature detected by thermistor 73.
Charging/discharging device 60 comprises: charging/discharging circuit 62 holding a resistor in its interior whereby discharging of battery pack 70 is performed and generating voltage/current used to charge battery pack 70 from the commercial power source supplied through a power plug 64, a charging/discharging control circuit 61 that effects control of charging in respect of battery pack 70 and control of discharging by pressing a discharging switch 63, terminal C71b connected with plus charging/discharging terminal C71a of battery pack 70, terminal C74b connected with minus charging/discharging terminal C74a, terminal C72b connected with type identification terminal C72a, and terminal C73b connected with temperature terminal C73a.
Thus, when charging/discharging of battery pack 70 is effected, terminals C71b-C74b of charging/discharging device 60 are connected to respective terminals C71a-C74a of battery pack 70 to perform charging/discharging. Thus the circuit is constituted such that charging/discharging current flows by connection of terminals C71a, C71b and terminals C74a, C74b respectively to the plus and minus terminals of the battery cell.
Terminal C72a detects the capacity type of battery pack 70: if battery pack 70 is of large-capacity type, this terminal C72a is connected to the minus terminal of battery cell 71 as shown in the drawing; if it is of small capacity, this terminal C72a is released from the minus terminal of battery cell 71.
Specifically, when this terminal C72a is connected to charging/discharging control circuit 61 through terminal C72b, charging/discharging control circuit 61 uses the signal from terminal C72b to detect whether this terminal C72a is connected to or is released from the minus terminal of battery cell 71, and, in accordance with the result of this detection, identifies the capacity type of this battery pack 70.
Also, terminal C73a is a terminal for detecting the temperature of battery pack 70 by detecting the current or voltage flowing in thermistor 73, and is connected to charging/discharging control circuit 71 through terminal C73b.
Charging is performed by connecting charging/discharging device 60 to battery pack 70. When battery pack 70 is connected, charging/discharging control circuit 61 identifies the capacity of battery pack 70 from the condition of terminal C72a and specifies to charging/discharging circuit 62 a charging current corresponding to the result of this identification; charging/discharging circuit 62 then generates the specified charging current and commences charging of the battery pack through terminals C71a, C71b and terminals C74a, C74b. Charging/discharging control circuit 61 then detects the temperature in the battery pack by means of the current through thermistor 73 and terminals C73a, C73b; if the detected temperature is more than a prescribed value, it instructs charging/discharging circuit 62 to cease charging; charging/discharging circuit 62 thereupon ceases charging. The battery pack can thereby be prevented from catching fire due to generation of heat etc.
Discharge is also effected by connecting charging/discharge device 60 to battery pack 70. However, when such discharge is performed, discharge switch 63 is depressed, so, in response to the depression of this discharging switch 63, charging/discharge control circuit 61 instructs connection of charging/discharging circuit 62 to a prescribed load resistor.
A closed circuit is thereby produced through the prescribed load resistor of charging/discharging circuit 62, terminals C71a, C71b, battery cell 71, and terminals C74a, C74b, and discharging is commenced. It may be remarked that the reason why discharging of battery cell 71 is performed is in order to eliminate the memory effect of the battery produced by repeated incomplete charging and discharging.
However, if a piece of metal or the like accidentally came into contact with terminals C71a-C74a of battery pack 70, current flowed from the plus terminal C71a through the piece of metal to the minus terminal C74a, or, if terminal C72a was connected to the minus terminal of battery cell 71, current flowed from the plus terminal C71a through the piece of metal to this terminal C72a, giving rise to the risk of generation of smoke from material in the vicinity or catching fire due to generation of heat by the piece of metal etc. Conventionally however this danger was forestalled by current cut-off by over-current cut-off switch 72 or detection of the temperature of the interior of battery pack 70 by means of thermistor 73.
As described above, if a large current flowed due to short-circuiting etc., the resistance of over-current cut-off switch 72 abruptly increased, thereby cutting off the current. The current value at which current was cut off by this over-current cut-off switch 72 was termed the "trip current".
However, if a metal object such as a chain accidentally comes into contact with terminals C71a-C74a of battery pack 70, since the resistance of a metal object such as this chain is several .OMEGA., a current of the order of several hundred mA, which is lower than the trip current, flows in the over-current cut-off switch 72.
As a result, generation of heat due to Joule heating by this current continues without actuation of over-current cut-off switch 72, with the result that the equipment may catch fire; there was therefore the problem that the risk of damage due to catching fire could not reliably be prevented.